Recent power conversion equipment is becoming higher in voltage and growing in size. In addition, equipment requiring capacitors of high voltage and large capacity have come into existence. In such equipment, a large number of capacitors are used by being connected in series, and in parallel. In a case in which the numbers of series connected capacitors and parallel connected capacitors become large, when a capacitor has partially fallen into a short circuit failure state (hereinafter, referred to simply as “failure”), this cannot be detected as a malfunction of the whole equipment, and if usage of the equipment is continued without any trouble-shooting, there may be a case that capacitors have failed one by one continuously, and the malfunction is unnoticed until a large amount of damage occurs. In order to prevent such expanded damage, usually, a capacitor failure detecting circuit is provided.
For this failure detecting circuit, a method for monitoring DC voltage is typically used, but when the numbers of the series connected capacitors and parallel connected capacitors increase, the problem as described above cannot be solved without providing a circuit to monitor voltage of each capacitor individually. Accordingly, a failure detecting circuit might become complicated.
On the other hand, proposals have been made in which intermediate points of the capacitors which are connected in series and parallel are connected by a diode bridge, a level detector to detect a voltage between the DC voltage terminals of the diode bridge is composed, and the failure of the individual capacitors is monitored by checking the output of this level detector (refer to Patent Document 1, for example).